Originally Posted by
safetypee
Gordon et al.
Would the scenario of a loose vane about the shaft fit all incidents.
Re Lion, the vane rotated on the shaft as far as 22 deg (grub screw); the vane aligns with the airflow, but misreads due to the repositioned shaft under the influence of the balance weight. This also enabled the vane to be checked by maintenance without fault, because the vane was still free to move around the shaft.
With Ethiopian, the vane became completely free to rotate during takeoff, then the balance weight positioned the shaft equivalent to full up ? (down?). The vane remained free in the airflow, but without connection it was irrelevant. The shaft remained free such that the balance weight reflected the effects of ‘g’ at the end of the flight.
Have the ‘errant’ AoA vane units been found; loose vane, or no vane.
Not sure why a common AoA failure cause is needed, in any case there are very significant differences between the 2.
Lion Air :Incorrect (offset) values present for entire (2!) flights, the offset shows as soon as speed is sufficient.
Not sure of aerodynamics but doubt that a damaged vane would have a consistent offset.
Since this was a refurbished unit it is possible that it was mis-assembled or damaged on installation/test, only other possibility is a wiring issue, which might fit with prior unit being intermittently bad. One can be sure the investigators will be looking at this in depth.
Ethiopian: Values ok at first then a sudden and dramatic shift to full over and at the same time the heater circuit opens.
This can be explained by bird strike or prior (ramp rash) damage.
Recovery of either unit intact enough for analysis is unlikely due to high energy of crashes and position of sensors.
Finding the missing vane is also unlikely due to small size and other factors, an all out search might find it but would at most just confirm one of the 2 causes, prior damage or bird strike.